Method of treating molten ferrous metals and materials for use in such treatment

ABSTRACT

THE PRESENT INVENTION IS A METHOD OF INTRODUCING RELATIVELY LOW BOILING METALS INTO MOLTEN FERROUS METALS WHICH COMPRISES ADSORBING THE VAPOR OF THE LOW BOILING METAL ON ACTIVATED CHARCOAL AND INTRODUCING THE METAL-ACTIVATED CHARCOAL ADDITIVE INTO THE FERROUS METAL MELT. THE INVENTION ALSO INVOLVES THE METAL-ACTIVATED CHARCOAL ADDITIVE AND METHOD OF ITS PREPARATION.

United States Patent Ofi ice Patented Aug. 10, 1971 3 598,572 METHOD OF TREATING MOLTEN FERROUS METALS AND MATERIALS FOR USE IN SUCH TREATMENT John Clifton Robertson, Lake Jackson, Tex., assignor to The Dow Chemical Company, Midland, Mich. N Drawing. Filed Nov. 22, 1968, Ser. No. 778,322 Int. Cl. C21c 1/10, 7/00 US. CI. 7553 10 Claims ABSTRACT OF THE DISCLOSURE The present invention is a method of introducing relatively low boiling metals into molten ferrous metals which comprises adsorbing the vapor of the low boiling metal on activated charcoal and introducing the metal-activated charcoal additive into the ferrous metal melt. The invention also involves the metal-activated charcoal additive and method of its preparation.

BACKGROUND OF THE INVENTION In many iron-making processes, it is desirable to introduce metals having a relatively low boiling point in relation to the melting point of iron into the iron melt. For example, nodular iron, also known as spheroidal graphite iron or ductile iron, is produced by the introduction of magnesium to the iron melt. The introduction of magnesium along with other higher boiling additives causes the carbon contained in the melt to coagulate into spheres. The resulting iron containing such dispersed spheres of carbon has improved strength properties. The introduction of other low boiling metals into an iron melt also has the beneficial efiYect of purifying the melt. For example magnesium and the alkali metals are useful as deoxidizers due to their ability to reduce dissolved oxides of the iron group and form insoluble metal oxides. These metals also attack the soluble sulfides of the iron group and form insoluble sulfides which may be removed from the melt. In a similar manner, finely dispersed silicates may be removed from the iron melt by the introduction of metals which form insoluble silicates.

The desirability of introducing magnesium and the alkali metals into the iron melt is apparent. However, great difiiculty is encountered in this procedure. The melting point of iron is much higher than the boiling points of the additive metals. For example, iron melts at about 2750 F. while magnesium boils at a temperature of about 2030 F. The introduction of pure or nearly pure magnesium directly into the iron melt causes the magnesium to flash into the vapor state. The metal vaporizes so rapidly that in many cases a reaction of explosive force results with the iron being ejected from the vessel in which it is being treated.

Several methods have been developed to alleviate the problem caused by the violent reaction which occurs when a low boiling metal is introduced into molten iron. One of these methods consists of alloying magnesium with higher boiling metals. The resulting alloy has a boiling point which is closer to the melting point of iron. In another method, a carrier gas, such as nitrogen, is used to force finely divided particles of magnesium through injection tubes into the molten iron. In another method, a wire or thin rod of magnesium is injected into the molten iron through a refractory tube. Still another method of introducing magnesium into molten iron is by the use of briquettes of magnesium and high melting metals, metallic oxides or refractories. All of these methods have met with some success, however, the expense involved in their use is appreciable.

Another method of introducing magnesium and other low boiling metals into an iron melt is that of Snow (US. 3,321,304). The Snow method consists of dipping porous metallurgical coke or porous graphite into a melt of the metal desired to be introduced into the iron melt and allowing the pores in the carbon to fill up with the metal. The metal impregnated carbon is then allowed to cool.

When the magnesium or other low boiling metal impregnated carbon is immersed into the iron melt there is a controlled rate of heat transfer into the metal resulting in a controlled rate of vaporization. This controlled rate of vaporization prevents the reaction from becoming too violent. After the low boiling metal vaporization has been completed, the carbon, a small amount of which has dissolved into the iron during the treatment, is withdrawn from the iron melt. This method is effective, however, the recovery of low boiling metal in the iron is only 20 to 50% in most cases due to its partial vaporization.

It is a principal object of the present invention to provide a material which may be used to introduce a relatively low boiling metal into molten iron without producing a violent reaction due to rapid vaporization of the low boiling metal.

It is another object of the present invention to provide such a material which may be easily and inexpensively prepared.

It is a further object of the present invention to provide such a material which, when introduced into molten iron will result in high recovery of the low boiling metal.

It is still another object of the present invention to provide a novel method for the introduction of a relatively low boiling metal into molten iron.

SUMMARY OF THE INVENTION The present invention comprises a metal having a boiling point below the melting point of iron adsorbed on activated charcoal and a method of introducing the metal into molten iron by use of this material.

Magnesium has a very high gas pressure at temperatures near that of molten iron, e.g., 12 atmospheres at 2700 F. This high gas pressure is responsible for the violent, sometime explosive, reaction which takes place when pure magnesium is introduced into molten iron. Similarly, other metals with boiling points substantially below the melting point of iron react violently when added to an iron melt. The present invention reduces the gas pressure of the low boiling metal appreciably.

Activated charcoal is permeated by a maze of exceedingly fine capillaries upon the walls of which adsorbed gas or vapor is held. When gases are adsorbed on these capillaries, their pressures are greatly reduced. Because of the great surface area of these capillaries per unit of charcoal, a small amount of charcoal reduces the pressure of a given amount of an adsorbed gas substantially.

Although the invention is not premised on an explanation of its underlying theory, it is believed that when a metal vapor is adsorbed on activated charcoal, a reduction of its gas pressure at a given temperature results.

In the practice of the present invention, vapors of the low boiling metal are adsorbed on activated charcoal, and the metal containing charcoal is added directly to the iron melt. Due to its adsorbed condition, the rate of vaporization of the loW boiling metal is effectively reduced. Depending upon the extent to which the metal atoms are dispersed within the charcoal, the gas pressure of the metal, at the temperature of molten iron, may be reduced to from slightly above 1 atmosphere to below atmospheric pressure at the temperature of molten iron. When the metal gas pressure is reduced to a level slightly above 1 atmosphere, the metal will vaporize at a controlled rate. When the gas pressure of the metal is reduced to below 1 atmosphere, the metal will dissolve into the iron melt along with the charcoal.

There are significant advantages to reducing the rate of vaporization or eliminating vaporization altogether of the low boiling metal in the described manner. One is that the problem of the violent reaction, which occurs when untreated low boiling metal is introduced directly into an iron melt, is obviated. Secondly, the recoveries of the low boiling metal are high due to relatively low vapor losses of the low boiling metal.

DETAILED DESCRIPTION OF THE INVENTION The preparation of the material of the present invention, i.e. low boiling metal adsorbed on charcoal, is accomplished by heating activated charcoal and the metal to the boiling point of the metal in a closed vessel, i.e. a furnace with closure. The process is carried out in a substantially inert atmosphere in order to prevent chemical reaction of the metal vapor. The absence of oxygen in the system is especially important due to the undesirability of formation of metal oxide. It is preferred to carry out the process in an evacuated system due to the lower temperatures at which the metal may be vaporized under vacuum. Alternatively, an inert gas, i.e. a gas inert to the metal being vaporized and the activated charcoal, may be introduced into the vessel. Normally the inert gas is introduced to the vessel and then removed to purge the vessel of oxygen or other non-inert gases.

Charcoal prepared from any carbonaceous material is useful in the practice of the present invention. The destructive distillation of wood, bone, shells, peat and lignite for example will provide a useful material. Since the charcoal is to be used to adsorb a vapor, a dense raw material such as coconut shells or soft coal is preferred as the starting material.

The resulting charcoal is activated by heating to a temperature of between 800 and 900 C. The material is not oxidized after the heating step as is done in some activation processes due to the undesirability of oxygen being present during the adsorption process. The charcoal may be activated before being charged to the furnace. Alternatively, the charcoal may be activated in situ. This is accomplished by adding the charcoal to the furnace and heating to between 800 and 900 C. The furnace is then evacuated and cooled. The metal is then charged to the furnace. The furnace is sealed and non-inert gases removed as hereinbefore described. The furnace is heated to at least the boiling point of the metal and allowed to remain at the elevated temperature for a time sufficient to allow substantial adsorption of the metal vapor on the charcoal. Adsorption is complete when an equilibrium is reached between the metal vapor adsorbing on and desorbing from the charcoal. After such equilibrium has been reached, which point maybe determined by a stabilization of pressure within the furnace, the system is allowed to cool.

After preparation, it is desirable to keep the material in an air-tight container until use to avoid oxidation of the metal.

The material prepared by the above-described method may be introduced into an iron melt by a conventional method.

The invention is normally practiced in the following manner:

Pieces of a low boiling metal are placed in a high temperature furnace. Above the metal is placed a tray of activated charcoal. The amount of low boiling metal used is preferably in excess of that required to substantially coat the adsorbing surfaces ofthe charcoal. The furnace is sealed and evacuated. The furnace is then purged of oxygen and other reactive gases by admitting an inert gas such as argon and re-evacuation. The furnace is then sealed and heated to a temperature suflicient to vaporize the metal. In the case of magnesium at 1 atmosphere pressure the furnace is heated to a temperature of at least 1107 C. This minimum temperature will vary depending on the boiling point of the metal being vaporized and the pressure conditions of the vessel.

Effective adsorption is achieved by maintaining the elevated temperature for a period of a few moments. Preferably, the elevated temperature is maintained for a time sufficient to reach an equilibrium between the metal being adsorbed by the charcoal and that being desorbed. This state is reached when the pressure inside the sealed furnace remains at a constant level. The furnace is then cooled. Pressure within the furnace is optionally maintained by the admission of an inert gas such as argon. The low boiling metal-charcoal additive is immediately placed in an air-tight container.

The above process may be practiced in a similar manner where it is desired to adsorb any relatively low boiling metal on activated charcoal. Magnesium and the alkali metals are preferred low boiling metals due to their usefulness in iron making processes.

In the production of iron, where it is desirable to in-- troduce a low boiling metal into the iron melt, the material is added by a conventional method such as plunging.

The invention may be practiced in a manner similar to the above method to introduce relatively low boiling metals into relatively high melting metals other than iron.

I claim:

1. A porous agent for the introduction of relatively low boiling metals into melts of relatively high melting metals which comprises activated charcoal upon whose surface and capillary walls have been adsorbed at least a significant amount of the vapor of the low boiling metal.

2. The agent of claim 1 wherein the low boiling metal is magnesium.

3. The agent of claim 1 wherein the low boiling metal is an alkali metal.

4. A process for preparing an agent for introducing relatively low boiling metals into melts of relatively high melting metals which comprises:

(a) contacting activated charcoal with the vapor of the relatively low boiling metal in a sealed vessel;

(b) maintaining a substantially inert atmosphere in the vessel;

(c) elevating the temperature of the vessel to a level sufiicient to keep the metal in the vapor state under the pressure conditions therein; and

(d) Allowing the activated charcoal and metal vapor to remain in contact for a period of time sufficient to allow a significant amount of metal vapor to adsorb on the activated charcoal.

5. The process of claim 4 wherein the relatively low boiling metal is magnesium.

6. The process of claim 4 wherein the relatively low boiling metal is an alkali metal.

7. The process of claim 4 wherein the activated charcoal and the metal vapor are allowed to remain in contact with each other under said conditions until an equilibrium is reached between the adsorbing and desorbing metal vapor.

8. The process of claim 4 wherein the atmosphere of the vessel is purged of oxygen with an inert gas and sealed before the temperature of the vessel is elevated.

5 6 9. A method for introducing a relatively low boiling 2,726,152 12/1955 Eash 75130 metal into a melt of a relatively high melting metal which 2,988,445 6/1961 Hurum 7558 comprises introducing the agent of claim 1 into 3,321,304 5/1967 Snow 7553X the melt. 3,384,463 5/1968 Olstowski 75168X 10. The method of claim 9 wherein the relatively high 5 3,393,996 7/ 1968 Robertson 7553 boiling metal is iron.

L. DEWAYNE R'UTLEDGE, Primary Examiner Refe'ences J. E. LEGRU, Assistant Examiner UNITED STATES PATENTS 1,594,347 8/1926 Bakken 75168X 2,231,023 2/1941 Nelson 7567 75123CB, 130A, 130B, 130C; 117-107; 164-57 

